The present invention generally relates to effectively reducing junction degradation in semiconductor devices, including those provided with a MOSFET, and more particularly to a semiconductor integrated circuit device capable of effectively reducing junction degradation caused by an increase in a leakage current at the p-n junction in a source region or a drain region of a MOSFET so that miniaturization of the semiconductor device can be expedited.
A conventional semiconductor device of the type disclosed in Japanese Patent Laid-Open No. 174721/1991 has been so arranged that the depth of the p-n junction is made shallow. In this particular device, the p-n junction is formed between a layer of one conductivity type which is a high carrier concentration layer containing impurities of not less than 10.sup.19 /cm.sup.3 and a layer of another conductivity type which is a low carrier concentration layer containing impurities of not more than 10.sup.18 /cm.sup.3.
In order to improve hot carrier immunity of MOSFETs, there have been employed in the past such arrangements as an LDD (Lightly Doped Drain) structure, as disclosed in Japanese Patent Laid-Open No. 242078/1986, and a double diffused drain structure, as disclosed in Japanese Patent Laid-Open No. 72272/1985. In these structures, the intended object has been accomplished by relaxing the electric field at the drain junction edge on the silicon surface.
Further, as disclosed in Japanese Patent Laid-Open Nos. 15465/1988, 280322/1990 and 62573/1991, a combination of LDD and double diffused drain structures has also been employed. The object of such a combination is to prevent an increase in junction leakage current due to dry etching or impairment of ion implantation for the high concentration layer formation by surrounding a damaged portion, resulting from the dry etching when the LDD structure is prepared, with a high concentration layer or a low concentration layer of the double diffused drain structure.
On the other hand, it has heretofore been employed to improve the junction breakdown voltage by relaxing the electric field biased to the p-n junction so as to make a carrier concentration profile on the high concentration side a graded junction as disclosed in Japanese Patent Laid-Open Nos. 188925/1990 and 201970/1990.
Further, as disclosed in Japanese Patent Laid-Open Nos. 244640/1990 and 177570/1990, an abrupt junction and a graded junction are combined in a plane fashion to effect the electric field relaxation on the periphery of the junction.
Still further, as disclosed in Japanese Patent Laid-Open Nos. 124713/1983 and 73669/1988, a high concentration buried layer of a conductivity type opposite to that of the source/drain diffused layers is formed in a portion deeper than the source/drain diffused layers to prevent punch through at the MOSFET.
The junction of a typical conventional semiconductor device has been formed near the semiconductor substrate surface where damage and metal contamination tend to occur frequently since they are introduced during the process of manufacturing such a semiconductor device, including ion implantation at the time of impurity doping or dry etching at the time of insulator processing.
In other words, the junction has such a concentration profile that the damage caused thereto and the degree of contamination tend to remain high, but decreases in proportion to its depth from the surface. Thus, if the position of the p-n junction formation is made shallow gradually from the semiconductor substrate surface as conventionally practiced, a depletion layer will contain a great deal of damage and contamination when the reverse bias voltage is biased to the p-n junction. Therefore, it is likely that the frequency of leakage current degradation of the p-n junction will become high.
Two known sources of leakage current are (1) leakage current through a generation-recombination center in a depletion layer, and (2) leakage current through a surface state in a portion where the depletion layer terminates at the semiconductor substrate surface. The present inventors have discovered that in addition to these two known sources, there also exists (3) a leakage current resulting from a local Zener effect generated at the periphery of a precipitate other than the semiconductor substance in the depletion layers generated when a p-n junction is reverse biased.
The leakage currents of the above-noted effects (1) and (2) are generated by the generation-recombination center and the surface state which are generated by the effects of damage and contamination introduced in the process of manufacturing the semiconductor. Thus, for example, if the p-n junction becomes shallow, the depletion layer also correspondingly becomes shallow, and the leakage current of the above effect (1) tends to increase. If the depletion layer of the p-n junction becomes wide, the volume of the depletion layer and the contact area with the semiconductor substrate surface increase correspondingly, and the leakage currents of the above effects (1) and (2) increase.
The leakage current of the above effect (3) is generated by a local Zener effect, which depends on the type of the precipitate. The Zener effect is generated at the periphery of the precipitate, and is generated by not only the effects of the above damage and contamination but also the effects of physical properties of the semiconductor substrate itself. Such a phenomenon will be described below.
If, in a semiconductor substrate, there exists a precipitate having a dielectric constant different from the dielectric constant of the semiconductor, a local electric field enhancement such as shown in FIG. 1 will be caused in a constant electric field. Typically, the term "contaminant" refers to a substance having a size on the atomic level (e.g., an ion), whereas "precipitate" refers to a substance having particles in a range typically between 50 .ANG. and 500 .ANG., although the invention is not limited to this. In FIG. 1, the local electric field enhancement in silicon is indicated. However, in the case of an oxygen precipitate, which can be regarded as an SiO.sub.2 ball, and a metal precipitate, which can be regarded as a metal ball, local electric field enhancements of at most 1.3 times and 3 times, respectively, are caused. Of course, various other precipitates are possible. These are determined by the types and degree of the damage and contamination introduced in the semiconductor production steps, or by the degrees of the oxygen concentration or point defect concentration of the semiconductor substrate itself to be used.
When such local electric field enhancement is caused, a band-to-band tunneling effect is locally caused, as disclosed in "A Theory of Electrical Breakdown of Solid Dielectrics", Proc. Roy. Soc., vol. A145 (1934), pp. 523, by Zener et al. A tunneling probability P (/s) in the case of a silicon substrate is represented by 1.35.times.10.sup.7 .times.E(V/cm).times.exp (-2.14.times.10.sup.7 /E(V/cm)) by a cgs unit system.
Thus, the tunneling probability in the case of the local electric field enhancement (called local Zener probability herein to distinguish it from usual tunneling probability) may be obtained by making the electric field E several times greater by the formula of the above tunneling probability P(/s). The magnifying power depends on the precipitate, as shown in FIG. 1. The present inventors have proved this phenomenon as follows.
At first, an oxygen precipitate and a metal precipitate were incorporated into a silicon substrate for the production of a p-n junction, and then an abrupt junction was prepared with a p-type low concentration (1.6.times.10.sup.16 /cm.sup.3) layer and an n-type high concentration (1.times.10.sup.20 /cm.sup.3) layer. A reverse bias was applied to the p-n junction and the leakage current was measured to see an increase as compared with the leakage current of the p-n junction when the above two types of precipitates are not introduced.
As a result, the leakage current increased by the introduction of the precipitate, as shown in FIG. 2. In this figure, the two auxiliary lines are the electric field dependency of the local Zener probability when the SiO.sub.2 precipitate and the metal precipitate were supposed. Namely, when the introduction of the precipitate is conducted, it is apparent that a leakage current increase by the metal precipitate is caused on the low electric field side and a leakage current increase by the SiO.sub.2 precipitate is caused on the high electric field side. Here, the incorporated amount and the incorporation distribution of the precipitate, are not controlled. However, this figure indicates the electrical field dependency of the leakage current increase corresponding to the dielectric constant of the precipitate intentionally incorporated. It is also evidenced that the band-to-band tunneling effect is locally caused in usual electric fields.
Taking into consideration the causes of the p-n junction degradation including the above-mentioned new leakage current, the conventional semiconductor devices have the following problems to be solved. First, in the LDD structure, since most regions of a source/drain junction of a MOSFET are a combination of a high concentration layer and a low concentration layer, if the junction depth is made challowor to be adapted for further miniaturization, the leakage currents of the above effects (1) and (3) tend to increase. In other words, when the damage and contamination introduced in the semiconductor production steps are distributed in such a manner that they tend to be very high at the surface and decrease with the depth, the generation-recombination centers increase corresponding to such an extent that the depletion layer is made shallow on the substrate surface. Thus, a major part of the electric field becomes closer to the portion in which a large amount of precipitates exist.
Next, in the double diffused drain structure, the leakage current of the above effect (3) can be reduced to the extent of relaxation of the electric field in the low concentration region. However, this is simply due to expanding the width of the depletion layer. As a result, the leakage currents of the above effects (1) and (2) will be increased correspondingly. Further, this double diffused drain structure has a problem of intrusion of a diffused layer beyond the gate electrode edge, and, therefore, is not suitable for miniaturization. Further, such a structure is not suitable for miniaturization from the viewpoint of controllability of the intrusion. Since the effect of the electric field relaxation in the double diffused drain structure is an electric field relaxation only at the high concentration layer side, a relaxation effect of about only 10% can be obtained as compared with the LDD structure.
In an arrangement combining the LDD structure and the double diffused drain structure, the increase of the leakage current of the above effect (3) can be prevented similar to the double diffused drain structure. However, the leakage currents of the above effects (1) and (2) will be increased similarly, which is, of course, unsuitable for miniaturization of the structure. A problem newly posed in this structure is that since the low concentration layer of the double diffused drain structure overlaps the low concentration layer of the LDD structure, it becomes difficult to control the concentration at the gate electrode edge as a measurement for hot carriers. That is, if the introduction of two different impurities results in respective scatterings, double scatterings will be caused. In this case, the electric field relaxation is only at the high concentration layer and therefore the obtained effect is only at the level of the double diffused drain.
Further, in a structure wherein the high concentration layer side is made a graded junction to improve the junction breakdown voltage and the hot carrier immunity, since the width of the depletion layer becomes large, similar to the case of the above double diffused drain structure, the leakage currents of the above effects (1) and (2) increase. Further, since the part to be depleted becomes closer to the substrate surface, not only the leakage currents of the above effects (1) and (2) but also the leakage current effect (3) are not ignorable in the part of the low electric field as well. To avoid this, there is only a method wherein the high concentration layer is formed in the deeper portion, such being unsuitable for miniaturization. In this connection, the obtained effect of the electric field relaxation in this case becomes large to such an extent that the depletion layer is widened. However, if it is practiced to adapt this structure to a miniaturization level, similar to the various above-mentioned structures, the obtained relaxation effect is eventually only about 10% as compared with the LDD structure, similar to the above structure.
There are not only structures having the depth direction distribution changed but also structures having the horizontal direction distribution changed from the plane viewpoint. However, in the latter structure, since the surface area of the so-called depletion layer is increased, the junction degradation due to the leakage current of the above effect (2) is increased, and since there are still junctions of a simple combination of the high and low concentration layers, it is impossible to avoid the p-n junction degradation by the leakage current of the above effect (3). Further, such a combination of junctions having different distributions from each other from the plane viewpoint is quite unsuitable for miniaturization.
Although a description has been given of distribution on the high concentration layer side, problems of conventional semiconductor devices with the distribution on the low concentration side changed will be described further.
In a structure having a high concentration buried layer which is provided at the deep part of the low concentration layer side to prevent the punch-through phenomenon in a MOSFET, the depletion layer of the above p-n junction is hardly widened by the effect of the high concentration buried layer. The smaller the depletion layer width at the same reverse bias, the larger the electric field in the depletion layer. By such a structure, the leakage current of the above effect (3) will increase. Further, the depletion layer will widen to the high concentration layer side at the surface side corresponding to such an extent that it is suppressed by the high concentration buried layer, and thus the leakage current of the above effect (1) will increase.
As mentioned above, the conventional semiconductor devices have problems that any one of the leakage currents of the above effects (1) to (3) which cause the p-n junction degradation, will increase. If the leakage currents of the above effects (1) and (2) are to be decreased, the leakage current of the above effect (3) will increase. If the leakage current of the above effect (3) is to be decreased conversely, the leakage currents of the above effects (1) and (2) will increase, such being unsuitable for miniaturization. Further, if the electric field relaxation occurs only at the high concentration layer side, the effect is about 10% at most, and thus only a small beneficial effect is obtainable against the reduction of the leakage current of the above effect (3).
Further, the conventional formation of the high concentration buried layer on the low concentration layer side leads to an increase of the electric field, and it is impossible to remove the effects of the electric field while adapting to miniaturization with respect to all of the conventional semiconductor devices mentioned above. Since no consideration was given in the prior art to the effect of precipitates and the effect of their distribution, no satisfactory measurement for the leakage failure of the p-n junction was practiced.